1. Field of the Invention
The present invention relates to an ink, and a recording method making use of it. More particularly, it relates to a water-based ink capable of providing an image having improved indoor color change resistance, and a recording method, in particular, ink-jet recording method, making use of it.
2. Related Background Art
Water-based inks comprising a water-soluble dye dissolved in a water-based medium have been hitherto as inks used in fountain pens and felt pens, and inks used for ink-jet recording. In these water-based inks, water-soluble organic solvents are commonly added so that pen points or ink ejection nozzles can be prevented from being clogged with ink.
It is required for these conventional inks, for example, to provide an image with a sufficient density, not to cause any clogging at pen points or nozzles, to have good drying properties on recording mediums, to cause less feathering, to have excellent shelf stability, and, particularly in ink-jet recording systems utilizing heat energy, to have excellent thermal resistance. It is also required for the image formed to have satisfactory lightfastness and waterfastness.
For example, for the most important inks, which are used in both monochromatic and full-color images, C.I. Food Black 2 has been mainly used taking account of the above required performances (see Japanese Patent Application Laid-Open No. 59-93766 and No. 59-93768).
Among the various required performances, what is particularly important is the fastness of the images formed.
In regard to the fastness of images, hitherto mainly questioned is the color fading due to direct sunlight or various illumination lights. Such a problem of color-fading has been attempted to be settled by the selection of dyes having superior lightfastness.
Recently, however, a problem of color changes of images has become significant in addition to the above color fading. Namely, images formed by conventional inks have not only the problem of color fading but also the problem of color changes. These give very significant influences on black inks used in a largest quantity. In particular, in the instance of full-color images, the color changes result in a great lowering of image quality, bringing about an significant problem for the formation of images.
This problem of color changes also occurs indoors without exposure to direct sunlight. The color changes are also accelerated depending on the types of recording mediums on which images are formed, and this problem of color changes has been unavoidable in respect of the C.I. Food black 2 that has been hitherto widely used.
So-called coated papers comprises a substrate such as paper and formed thereon an ink-receiving layer containing a pigment and a binder, for the purpose of improving the color-forming performance of ink and the image quality such as sharpness and resolution. In such an instance, the color changes may seriously occur even with use of inks that may cause less problem of color change on plain papers. This problem has not been resolved by the mere selection of dyes having superior lightfastness.